1. Field of the Invention
This invention relates to synchronous networks, and more particularly, to the transmission of digital information over a synchronous network.
2. Description of the Related Art
Network transmission of data, video, images and/or voice using synchronous digital multiplex network techniques is well known. In general, transmission of digital information over the network is accomplished with a bitstream. The bitstream is produced from a source coupled to the network at an input node and is received by a sink coupled to the network at an output node.
Synchronous transmission transmits digital information in the form of network frames separated by equal time intervals. The time intervals and the network frames are fixed at a network master clock rate generated by a network master clock. Generally, synchronous transmission relies on finely controlled timing that is synchronized between the source, the network and the sink. Conversely, asynchronous transmission is non-time dependent in the sense that digital information can be transmitted at random intervals. In asynchronous mode, the digital information is coded with start bits and stop bits to indicate the beginning and end of segments of the digital information. Isochronous transmission techniques are time-dependent, however, the time-dependency provides more flexible time constraints for data transmission than the fixed time intervals of synchronous transmission.
One significant benefit of synchronous transmission over asynchronous and isochronous transmission techniques can be the minimization of uncertainty in timing between the source and the sink. Minimization of timing uncertainty minimizes irregularities in the transmitted signal typically referred to as “jitter.” Jitter manifests itself as audible irregularities in audio transmissions and vibration or fluctuations of display images in video transmissions.
In synchronous networks, operation of the source and the sink are synchronized with the frequency of the network master clock. Where the sample rate (or sample frequency) of the digital information processed by the source or the sink is different from the network master clock rate, the sample rate is converted. A sample rate converter is used to convert the sample rate to the frequency of the network master clock. Accordingly, sample rate converters are needed for each source and/or each sink that operate with a sample rate different from the network master clock rate.
For synchronous networks that include multiple sources and sinks, individual sources and sinks may be operating at various sample rates higher and/or lower than the network master clock frequency. As such, significant numbers of sample rate converters may be needed to convert to, and from, the network master clock frequency. For each sample rate converter, additional circuitry and wiring is required thereby increasing the cost and complexity of the network.